Hot-dip galvanized steel sheets have been widely used in automobiles, building materials, various structures, and household appliances due to having excellent corrosion resistance properties, and in particular, the high strengthening of steel sheets has been continuously undertaken in line with recent demands for weight reductions in vehicles. However, since the ductility of steel sheets relatively decreases when the strength thereof is increased, high-strength steels having improved ductility, such as dual phase (DP) steels having manganese (Mn), silicon (Si), or aluminum (Al) added to a base steel sheet, complex phase (CP) steels, and transformation induced plasticity (TRIP) steels, have been manufactured.
However, Mn, Si, or Al, added to steel sheets, may react with a trace of oxygen existed in an annealing furnace to form a single or complex oxide of Si, Mn, or Al, and thus, bare spots may be generated to degrade surface qualities of the plated steel sheet.
As a typical method for addressing the foregoing limitations, Japanese Patent Application Laid-Open Publication No. 2005-200690 discloses a technique in which a base steel sheet is coated with metal, such as nickel (Ni), after annealing and cooling to cover Mn oxide, Si oxide, or Al oxide formed on a surface thereof during annealing with the metal coating layer. In general, a continuous hot-dip galvanizing process is integrally configured in order to maintain a reducing atmosphere from an annealing process to a galvanizing process. However, with respect to the above technique, the annealing and galvanizing processes must be separated in order to allow for an annealing process before the metal coating process, and thus, a manufacturing facility may be complicated and manufacturing costs may be increased.
As another typical method for addressing the foregoing limitations, there is provided a technique, in which metal coating is performed in advance, and annealing and plating are subsequently performed. However, in the case that a temperature of 750° C. or above is used during annealing, the coated metallic materials may be diffused into a base steel sheet to this dissipate or thin a metal coating layer, and thus, there may be limitations in substantially preventing the surface diffusion of Mn, Si, or Al.
Therefore, the need for a economical technique, allowing for the surface qualities of a plated steel sheet to be improved by preventing the generation of bare spots through the inhibition of the formation of Mn oxide, Si oxide, or Al oxide on the surface of the steel sheet, has rapidly increased.